Marine propulsion systems today consist of an engine, a forward/reverse transmission, possibly a reduction gear assembly and a propeller assembly. Variation in the speed of the vessel is brought about by changing the engine speed. The range of speed of the vessel is directly related to and limited by the range of speed of the engine.
There are advantages and disadvantages to the use of gasoline and diesel engines to propel boats. When compared against each other, diesel engines are more dependable, are more efficient, use fuel that is less dangerous, as well as less expensive, have a longer life, and require fewer parts and maintenance. Their only disadvantage is that they are not capable of sufficiently high speed to drive boats as fast as gasoline engines of comparable power, with current propeller drive systems.
The range of engine speed of gasoline engines which gives them the ability to propel boats faster than diesel engines is their single advantage. This is often the consideration which overrides all others in the selection of engines.
Gasoline engines have been the best choice for marine applications when a broad range of vessel speed is desired. This is simply because gasoline engines offer an engine speed range of 2 to 2.5 times that of a diesel engine. In addition, the cruising engine speed of the diesel engine is a much greater percentage of its overall engine speed with current drive configurations than the gasoline counterpart and therefore the fuel economy advantage is not as great.
Most marine propulsion systems have single propeller drives or dual propeller drives. The propeller is turned by the engine to move the water stream one way while the vessel is pushed in the other. Propellers are made of several axially inclined planes that turn with the propshaft by force of the engine. Propellers are measured by diameter and pitch. The pitch is determined by the angle of the inclined planes and measured by the distance the propeller would travel in a semisolid material in one revolution. The larger and greater degree of angle perpendicular to the propshaft of the propeller blades, the greater the thrust delivered per revolution. As the propeller turns, the water is forced in the direction opposite that you desire to drive the vessel by the inclined planes. The displaced water leaves an area of low pressure ahead of the blades which helps to pull the propeller in the desired direction.
Variable pitch propellers, either mechanically or centrifugally operated, and variable speed transmissions are the two means now available to bring about changes in the speed of vessels without changes in the engine speed. Mechanically operated variable pitch propellers are devices which permit the operator to change the pitch of the blades of the propellers, thereby increasing or decreasing the speed of the vessel without changing the speed of the propeller. When such propellers are centrifugally operated, a substantial change of propeller speed is required to bring about a change in pitch.
Variable pitch propellers, mechanically operated, are too expensive to be practical. Variable pitch propellers, centrifugally operated, used with gasoline engines, will not work with diesel engines because diesel engines are not capable of the broad range of engine speed that they require to actuate the apparatus which changes their pitch. Propellers driven by variable speed transmissions operated at high speeds tend to have any increased thrust offset by cavitation which rapidly damages the propeller.
Dual counter-rotating propeller systems are being used more and more today. They allow for improved slow speed performance because two propellers engage more water than does one. However, at high speeds the rearward propeller runs in the wash of the forward one and little or no speed range increase is obtained. In addition, the differential apparatus which provides for the counter rotation absorbs energy in its own right.
Coaxially aligned propellers for use with marine engines are well known in the art. Most propulsion systems having coaxially aligned propellers have one large driven propeller and one small free wheeling propeller. Such designs dating from 1906-1986 are shown and described in the U.S. Pat. Nos. to Gray 811,287, Cake 1,224,120, Kubota 3,549,271, and Grim 4,623,299.
Other types of boat propeller systems are disclosed in the U.S. Pat. Nos. to Dalen 888,390, Taylor 938,911, Pierce 1,910,561, DeMichelis 2,064,195, Naginskas 2,196,706, Pleuger 3,127,865, Nohara 4,642,059, Brandt 4,767,269, Kouda et al. 4,828,518, McCormick 4,832,636, and Japanese Patent 63-34295. These patents disclose propulsion systems which attempt to drive a marine vessel with greater efficiency. Varying levels of thrust are not addressed by any of the patents of the prior art. This invention is designed to place diesel engines in a position to drive a boat as fast or faster than a gasoline engine of comparable power, using diesel engines with the range of engine speeds being manufactured today, and allowing low speed operation without using a "trolling valve" or similar propeller shaft speed reduction device.